High modulus fibers such as fiberglass are commonly used as resin reinforcements in composite materials to impart strength to the cured article. In some applications the fiberglass yarn is woven or knit into a fabric which can then be treated with a finish, resin or other coating. Compared to organic fibers such as polyester, glass fibers exhibit virtually no elongation and are more prone to breakage during process operations such as knitting and weaving. Because the fiber elongation is essentially nil, glass fabrics do not stretch unless they are constructed with very loose loops which can deform upon application of tension, thereby giving the impression of stretching. Knitting imparts extensibility by virtue of its system of interlocking knots and loops.
An important usage of knitted fiberglass fabrics is in the manufacture of orthopedic cast bandages where those fabrics are coated or impregnated with a curable resin and packaged as rolls of tape, which are subsequently used by a physician or medical clinician to construct orthopedic casts. When constructing an orthopedic cast from a roll of cast bandage (typically four-inches wide), it is necessary to make tucks or folds to accommodate excess material when changing the direction of wrapping or when going around contours such as the heel of the foot. This requires a high level of skill on the part of the applier to achieve a functional and smooth cast. To eliminate tucks and folds and facilitate application, it is desirable that the bandaging material be extensible. At least 20% and preferably 25%-35% extensibility is necessary in material of four-inch width to conform easily around the heel of a medium size adult.
Prior to the present invention, most commercially-available, resin-coated, knitted fiberglass casting tapes exhibited approximately 5%-15% extensibility beyond their relaxed length. The only product which provided greater extensibility, a product marketed under the trademark "K-Cast" (manufactured by Hexcel Corporation, San Francisco, Calif.), suffered from the problem of fraying along cut edges.
Like most knitted fabrics, fiberglass knits tend to curl or fray at a cut edge as the yarns are severed and adjacent loops unravel. When a segment of yarn is removed from such a fabric and allowed to relax, it remains in the new position in which it is placed. Fraying and raveling produce unsightly ends and, in the case of an orthopedic cast, frayed ends may interfere with the formation of a smooth cast, and loose, frayed ends may be sharp and irritating. Accordingly, frayed edges are considered a distinct disadvantage in orthopedic casting tapes.
It is well-known that fraying of fiberglass knits at cut edges can be prevented by passing the fabric through a heat cycle which sets the yarns giving them new three-dimensional configurations based on their positions in the knit. Fiberglass fabrics intended to be coated or impregnated with a resin are sometimes put through a heat-setting process for the additional purpose of removing sizing from glass filaments which would otherwise interfere with the proper application of the resin.
A batch process recommended by Owens-Corning Fiberglass Corporation, Toledo, Ohio to remove the sizing on the glass filaments results in a heat-set fabric and consists of the following steps:
1. Start at 220.degree. F. for 1.5 hours;
2. Raise to 480.degree. F. over a period of 5 hours and hold for 12 hours;
3. Raise to 700.degree. F. over a period of 3 hours and hold for 33 hours;
4. Cool to room temperature.
A continuous process involving much less time is also possible.
When a fiberglass fabric which has been heat-set is cut, there is minimal fraying and when a segment of yarn is removed from the fabric and allowed to relax, it curls into the crimped shape in which it was held in the knit. Accordingly, at the site of a cut, the severed yarns have a tendency to remain in their looped or knotted configuration rather than to spring loose.
Thus, the problem of alleviating frayed ends is easily overcome by conventional heat-setting processes. However, as one aspect of the present invention, it has been discovered that conventional heat-setting processes significantly reduce extensibility of fiberglass knits. Thus, prior to the present invention, resin-coated, heat-set (and therefore non-fraying) fiberglass knit tapes exhibiting extensibility of at least 20 percent were not available.